CMOS image sensor with improved performance incorporating pixels with burst reset operation

ABSTRACT

A reset transistor includes a floating diffusion region for detecting a charge, a junction region for draining the charge, a gate for controlling a transfer of the charge from the floating diffusion region to the junction region upon receipt of a reset signal, and a potential well incorporated underneath the gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority of Korean patent applicationnumber 10-2006-0040712, filed on May 4, 2006 which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a solid-state image sensor, moreparticularly to a complementary metal oxide semiconductor (CMOS) imagesensor improved in performance with high charge well capacity, whichresults in a higher dynamic range (DR) and an enhanced signal to noiseratio (SNR).

A typical pixel of the CMOS image sensor includes a photodiode, moreprecisely a pinned photodiode, and four transistors. The photodiodecollects a photo-generated charge that is later transferred at asuitable moment by a charge transfer transistor onto a floatingdiffusion (FD) node which detects the charge. Prior to the chargetransfer, however, the FD node must be first reset by a reset transistorto a suitable reference voltage, which is usually a terminal biasvoltage, Vdd, or a bias close to this level. The reset causes kTC noise,which is normally added to a signal appearing on the FD node. Thus, itis necessary to read a voltage on the FD node twice, before and afterthe charge transfer. This operation is called a correlated doublesampling (CDS) procedure and the CDS procedure allows sensing of onlythe voltage difference of the FD node caused by the charge transferredfrom the photodiode. A source follower (SF) transistor includes a gatecoupled to the FD node, a junction node coupled to the Vdd, and anotherjunction node coupled to a common column sense line via an addressingtransistor, and senses the voltage of the FD node. For this reason, itis typical to incorporate the four transistors in each pixel of astandard CMOS image sensor. The four transistors (4T) pixel circuit withthe pinned photodiode is proposed in U.S. Pat. No. 5,625,210 issued toLee, entitled “Active Pixel Sensor Integrated with a Pinned Photodiode”.

In modern CMOS sensor designs, a circuit for several photodiodes may beshared as can be found for an example in U.S. Pat. No. 6,657,665B1issued to Guidash, entitled “Active Pixel Sensor with Wired FloatingDiffusions and Shared Amplifier”. In this patent, a dual pixel includestwo photodiodes located in the neighboring rows of a sensor image arrayand sharing the same circuit. The shared photodiode concept can resultin having only two metal bus lines in the row direction and two metalbus lines in the column direction per photodiode as shown in FIG. 1. Theshared photodiode concept is very useful for designing small pixelssince spacing and width of the metal lines determine a minimum pixelsize. The operation of the shared photodiode pixel circuit can be easilyunderstood from a simplified schematic diagram of the circuit 100 shownin FIG. 1. Two photodiodes 101 and 102 are coupled through therespective charge transfer transistors 103 and 104 to a FD node 114which detects charge. The FD node 114 is reset by a reset transistor 105to a voltage level appearing on a Vdd node 108. The Vdd node 108 iscoupled to a column Vdd bus line 115. Gates of the charge transfertransistors 103 and 104 are biased through the respective horizontal buslines 110 and 111, and a gate of the reset transistor 105 is biasedthrough a horizontal bus line 109. A gate control signal Tx1 of thecharge transfer transistor 103 is transferred through the horizontal busline 110, and a gate control signal Tx2 of the charge transfertransistor 104 is transferred through the horizontal bus line 111. Also,a gate control signal Rx of the reset transistor 105 is transferredthrough the horizontal bus line 109. The FD node 114 is coupled to agate of a source follower (SF) transistor 106 that senses a voltage onthe FD node 114. An output of the SF transistor 106 is coupled throughan addressing transistor 107 to a common column sense line 113. Ahorizontal bus line 112 controls a bias of the addressing transistor107. A gate control signal Sx of the addressing transistor 107 istransferred through the horizontal bus line 112. As shown in FIG. 1,there are only two vertical (column) lines, a Vdd bus line 115 and acolumn sense line 113. There are also only two horizontal lines perphoto site.

It is possible to eliminate the charge transfer transistor from the 4Tpixel and thus, obtain a three transistors (3T) pixel. Also, it ispossible to eliminate one of the charge transfer transistors 103 and 104from the shared pixel shown in FIG. 1 and keep the other one in place,thereby obtaining a combination of a 3T-4T shared pixel. However, theCDS procedure may not be used to minimize the reset noise in the 3Tpixel and thus, other techniques known in the art have been required.

In order to better understand the operation of the 4T pixel, FIG. 2illustrates a cross-sectional view of a simplified device. Thesimplified device 200 includes a single pinned photodiode 205, and acharge transfer transistor and a reset transistor corresponding to thephotodiode 205. A source follower (SF) transistor 201 and an addressingtransistor 202 are shown only schematically including connections of theSF transistor 201 and the addressing transistor 202 to the correspondingcircuit nodes. A junction node of the SF transistor 201 is coupled to abus line 231. FIG. 2 also includes a potential diagram 206 of thesimplified device 200, and the potential diagram 206 illustrates acharge transfer flow from the photodiode into a FD node 203 and finallyinto an N⁺-type impurity doped junction region 204 (in this case,functioning as a drain) during charge reset. A pixel is formed in aP-type silicon substrate 219 that has pixel isolation trenches 207formed in the P-type silicon substrate 219 and filled by an oxide 210.Another oxide layer 218 is grown on a top portion of the substrate 219that isolates a gate 211 of a charge transfer transistor and a gate 212of a reset transistor from the substrate 219. The gates 211 and 212 ofthe charge transfer transistor and the reset transistor are coupled tothe respective horizontal bus lines 213 and 214 that supply a requiredbias to the gates 211 and 212. Gate control signals Tx and Rx of thecharge transfer transistor and the reset transistor are transferredthrough the horizontal bus lines 213 and 214, respectively. The pinnedphotodiode 205 is formed in the substrate 219 by a P⁺-type diffusionregion 208 that extends along a sidewall of each of the trenches 207 allthe way to the P-type substrate 219 and by an N-type diffusion region209. It is also possible to use more sophisticated doping profiles as iswell known to those skilled in this art. The charge transfer transistorwith the gate 211 connects the pinned photodiode to the FD node 203, andthe reset transistor with the gate 212 connects the FD node 203 to thejunction region 204 doped with the N⁺-type impurity.

As can be seen from the potential diagram 206 that is located under thecross-sectional view of the simplified device 200 in FIG. 2, potentiallevels of the potential diagram 206 correspond to each device built intothe substrate 219, and the pinned photodiode forms a potential well thataccumulates a signal charge 221 during the time when the gate 211 of thecharge transfer transistor is off. The off state of the gate 211 of thecharge transfer transistor is indicated in the potential diagram 216 bya potential level 222. An overflow charge (blooming current) from thepotential well flows via a path 230 through the reset transistordirectly to the junction region 204. When the charge transfer transistoris turned on, the potential level 222 under the charge transfertransistor changes to a potential level 223, and the signal charge 221flows into the FD node 203. The signal charge transferred into the FDnode 203 is indicated by a reference numeral 224. The signal charge 224causes a potential level 225 of the FD node 203 to change to a newpotential level 226. The new potential level 226 represents a voltagesignal that is sensed by the SF transistor 201. When applying anappropriate bias, i.e., a gate control signal Sx, to a gate of theaddressing transistor 202 through a bus line 215, the addressingtransistor 202 is turned on and the voltage signal from the SFtransistor 201 is transferred to a pixel output bus 216.

The pixel is reset by applying a single reset pulse 217 to thehorizontal bus line 214 coupled to the gate 212 of the reset transistor.The pixel reset changes a potential level 227 under the reset transistorto a new potential level 228, which allows the signal charge 224 to flowinto the junction region 204 doped with the N⁺-type impurity. Thejunction region 204 is biased at a Vdd potential level 229, which isfixed by an external device power source and cannot change. Thus, whenall the signal charge 224 is transferred to the junction region 204, theFD node 203 resumes the original potential level 225 of the FD node 203.The potential level 225 of the FD node 203 is only approximately equalto the Vdd potential level 229 due to a reset feed through from the gate212 of the reset transistor and kTC-reset noise. It is thereforedesirable to use the CDS procedure to sample both of the potentiallevels 225 and 226 since a true photo-generated signal is the differencebetween these two potential levels.

As is now clear from the potential diagram 206, a FD voltage swing islimited on a high side by the Vdd potential level 229 and on a low sideby an empty pinned photodiode level 220 of the pinned photodiode. Whentoo much signal accumulates in the photodiode, not all of the signal canbe transferred into the FD node 203, which results in the lower dynamicrange and increased noise. It is desirable to have a large voltage swingon the FD node 203 and thus decrease the pinned photodiode level 220,i.e., a pinning voltage level of the pinned photodiode. However, the lowpinning voltage level results in low photodiode charge storage capacity,thereby causing a low dynamic range (DR) and a low signal to noise ratio(SNR). Accordingly, it is necessary to find a suitable compromisebetween these two competing requirements. This compromise thendetermines an ultimate performance of the pixel that is clearlydetermined by the maximum Vdd potential level 229 that can be used inthe sensor. To have a large Vdd potential level is a disadvantage, sincethe large Vdd potential level increases the sensor power consumption.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to provide a practicalpixel design that can be used in a small pixel and a CMOS image sensorarray with a high performance.

In accordance with one aspect of the present invention, there isprovided a reset transistor, including: a floating diffusion region fordetecting a charge; a junction region for draining the charge; a gatefor controlling a transfer of the charge from the floating diffusionregion to the junction region upon receipt of a reset signal; and apotential well incorporated underneath the gate.

In accordance with another aspect of the present invention, there isprovided a pixel of an image sensor, the pixel including: a photodiode;a resetting device comprising a reset transistor, the reset transistorincluding: a floating diffusion region for detecting a charge; ajunction region for draining the charge; a gate for controlling atransfer of the charge from the floating diffusion region to thejunction region upon receipt of a reset signal; and a potential wellincorporated underneath the gate.

In accordance with further aspect of the present invention, there isprovided a shared photo site pixel, including: a photodiode coupled to afloating diffusion region in parallel; and a reset transistor forresetting the floating diffusion region, the reset transistor including:a junction region for draining a charge; a gate for controlling atransfer of the charge from the floating diffusion region to thejunction region upon receipt of a reset signal; and a potential wellincorporated underneath the gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified schematic diagram of a typicalrow-shared pixel circuit having two pinned photodiodes per circuit andoperating based on a four transistors (4T) pixel principle.

FIG. 2 illustrates a simplified schematic diagram of a typical pixelincluding a potential diagram showing a flow of an accumulated signal,and a cross-sectional view of a photodiode, a charge transfertransistor, and a reset transistor.

FIG. 3 illustrates a simplified schematic diagram of a pixel including apotential diagram showing a flow of an accumulated signal, and across-sectional view of a photodiode, a charge transfer transistor, areset transistor, and a potential barrier adjacent to a junction regionin accordance with an embodiment of the present invention.

FIG. 4 illustrates a simplified potential diagram showing a circuitmodel corresponding to an adiabatic reset which reduces kTC noise inaccordance with another embodiment of the present invention.

FIG. 5 illustrates a simplified schematic diagram of a standardisothermal reset which exhibits normal kTC noise in accordance withanother embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

According to embodiments of the present invention, a reset concept doesnot limit a floating diffusion (FD) node which detects charge to bereset at a Vdd bias level but at a level which is significantly abovethe Vdd bias level. The reset is performed by adding N burst resetpulses instead of adding a single reset pulse to a reset transistor,wherein N is greater than 1. Furthermore, the reset transistor can bemodified to include a potential well underneath a gate of the resettransistor. As a result, a junction region of the reset transistor isnot directly coupled to the Vdd but via a potential barrier. Thismodification of the reset transistor allows the charge to be pumped upfrom the FD node to a level above the Vdd bias level and thus, achievemuch higher reset voltage level than the Vdd. In addition, according tothe embodiments of the present invention, a charge-pumping concept whichreduces the kTC noise is shown. A typical reset can be compared with anisothermal equilibrium of the FD node having a charge reservoir, whichgenerates kTC noise. The new charge-pumping reset according to theembodiment of the present invention can be compared with an adiabaticcharge transfer as a reservoir which has a reduced level of noise. Thenew charge-pumping reset is very advantageous when a three transistors(3T) pixel is used or a combination of shared 3T-4T pixels is used in animage sensor array.

According to the embodiments of the present invention, the burst of thereset pulses is used instead of the single reset pulse transferred tothe gate of the reset transistor. To use the burst of the reset pulsesallows the FD node to be reset at a level significantly above the Vddbias level and thus, simultaneously increase a photodiode charge storagecapacity level and a FD voltage swing. The increase in the photodiodecharge storage capacity and the FD voltage swing can be obtained byincorporating a special potential well underneath the gate of the resettransistor and a special pinned potential barrier between the resettransistor and a Vdd node doped with an N⁺-type impurity. As the resultof the embodiments of the present invention, a complementary metal oxidesemiconductor (CMOS) sensor array can have a very small sized pixel, alarge charge storage capacity, a high dynamic range, and a high signalto noise ratio. Hereinafter, the embodiments of the present inventionwill be described in more detail with reference to FIGS. 3 to 5.

FIG. 3 illustrates a cross-sectional view of a simplified device inaccordance with an embodiment of the present invention. The simplifieddevice 300 includes a single pinned photodiode structure 305, and acharge transfer transistor and a reset transistor corresponding to thepinned photodiode structure 305. A source follower (SF) transistor 301and an addressing transistor 302 are shown only schematically includingconnections of the SF transistor 301 and the addressing transistor 302to the corresponding circuit nodes. An N⁺-type impurity doped junctionregion 304, which functions as a drain according to the presentembodiment, and a junction node of the SF transistor 301 are coupled toa bus line 331 to supply a bias Vdd. The simplified device 300 alsoincludes a potential diagram 306. The potential diagram 306 illustratesa charge transfer flow from a photodiode well into a FD node 303 andfinally into the junction region 304 during the charge reset. The pixelis isolated by trenches 307 formed through a shallow trench isolation(STI) process in a P-type silicon substrate 319. The trenches 307 arefilled with an oxide 310. Another oxide layer 318 is grown on a topportion of the substrate 319 that isolates a gate 311 of a chargetransfer transistor and a gate 312 of a reset transistor from thesubstrate 319. The gate 311 of the charge transfer transistor and thegate 312 of the reset transistor are coupled to the respectivehorizontal bus lines 313 and 314 that supply the required biases to thegates 311 and 312. Gate control signals Tx and Rx of the charge transfertransistor and the reset transistor are transferred through thehorizontal bus lines 313 and 314, respectively. The pinned photodiode305 is formed in the substrate 319 by a P⁺-type diffusion region 308that extends along a sidewall of each of the trenches 307 all the way tothe P-type substrate 319 and by an N-type diffusion region 309. Thecharge transfer transistor with the gate 311 connects the pinnedphotodiode 305 to the FD node 303 and the reset transistor with the gate312 connects the FD node 303 to the junction region 304 via a potentialbarrier, more particularly, a pinned potential barrier including aP⁺-type diffusion region 334 and an N-type diffusion region 335. Thispinned potential barrier is a junction gate field-effect transistor(JFET) including a gate coupled to a substrate, one junction regionbeing virtual, and another junction region shared with the junctionregion 304 of the reset transistor. The reset transistor has anadditional N-type implantation region 336 (a doped region) formedunderneath the gate 312 of the reset transistor, thereby forming apotential well. The potential well is formed on the side of the junctionregion 304 inside the substrate 319 underneath the gate 312 of the resettransistor. The JFET and the pinned photodiode 305 can be simultaneouslyformed during the fabrication process and thus, pinned potential levels320 and 333 can be substantially the same.

As can be seen from the potential diagram 306 shown under thecross-sectional view of the simplified device 300, potential levelscorresponding to each device that is built into the substrate 319 areshown. The pinned photodiode 305 forms a pinned photodiode potentialwell 320 that accumulates a signal charge 321 during the time when thegate 311 of the charge transfer transistor is turned off. The off stateof the gate 311 of the charge transfer transistor is indicated in thepotential diagram 306 by a potential level 322. An overflow charge (ablooming current) from the potential well 320 flows via a path 330directly to the junction region 304. When the gate 311 of the chargetransfer transistor is turned on, the potential level 322 under the gate311 of the charge transfer transistor changes to a potential level 323,and the signal charge 321 flows into the FD node 303 doped with anN⁺-type impurity. The signal charge transferred into the FD node 303 isindicated by a reference numeral 324. Accordingly, a potential level 325under the FD node 303 is changed to a new potential level 326. The newpotential level 326 is a voltage signal sensed by the SF transistor 301.When applying an appropriate bias to the gate of the address transistor302 through a bus line 315, the address transistor 302 is turned on, andthe voltage signal is transferred from the SF transistor 201 to a pixeloutput bus line 316.

According to the embodiment of the present invention, a new pixel isreset in the different way from the reset of the typical pixel. The newpixel is reset by applying a burst of rest pulses 317 to the bus line314, which transfers the gate control signal Rx to the gate 312 of thereset transistor. The burst of the reset pulses 317 periodically changesa potential level 327 underneath the gate 312 of the reset transistor toa new potential level 328 and vice versa. This process allows a portionof the signal charge 324 to flow into a potential well 332 formedunderneath the gate 312 of the reset transistor by placing a suitableimplantation region 336 underneath the gate 312 of the reset transistor.The potential well 332 is filled up and a potential level of thepotential well 332 is raised above a potential level 333 of the pinnedpotential barrier during a negative reset pulse transition. Accordingly,the signal charge in the potential well 332 under the gate 312 of thereset transistor flows out over the pinned potential barrier and intothe junction region 304, which is at a potential level 329. Thesufficient repetition of this process empties the signal charge of theFD node 303 and brings the potential level of the FD node 303 back tothe approximately original potential level 325. Since the junctionregion 304 remains biased at an original Vdd level 329, it is clear fromthe diagram 306 that the FD node 303 can be reset at a level above theVdd level 329 when the amplitude of the reset pulses and the depth ofthe potential well of the gate 312 of the rest transistor are suitablyselected.

As is now clear from the potential diagram 306, the FD voltage swing canbe higher and not be limited on the high side by the Vdd level 329. Thelow side remains limited by the potential level 320, i.e., an emptypinned photodiode potential level. However, the pinned photodiode level320 can now be significantly increased in comparison to the typicalpinned photodiode level. As a result, a charge storage capacity leveland a voltage swing of the FD node 303 can be increased, and a pixelperformance can be significantly improved without any significantpenalty to the power consumption of an image sensor or a silicon chiparea.

The burst-reset concept has another advantage of low reset noise. FIG. 4illustrates a simplified potential diagram showing a circuit modelcorresponding to an adiabatic reset which reduces kTC noise inaccordance with another embodiment of the present invention. As shown inFIG. 4, a charge-pumping operation from a potential level 428 to apotential level 427 and the potential diagram can be represented by thecircuit model consisting of a floating diffusion capacitance C_(fd) 401coupled through a switch 403 to another smaller capacitance C_(f) 402representing a rest transistor well 432. As is well known in the art,the reset noise appearing on the floating diffusion capacitance C_(fd)401 of the circuit configuration after a repeated opening and closing ofthe switch 403 is expressed by the following equation 1.

$\begin{matrix}{V_{n} = {\sqrt{\frac{kT}{C_{fd}}}\sqrt{\frac{C_{w}}{C_{fd} + C_{w}}}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

FIG. 5 illustrates a simplified schematic diagram of a standardisothermal reset which exhibits normal kTC noise in accordance withanother embodiment of the present invention. For the case of the C_(fd)reset to a voltage source as shown in FIG. 5, the reset noise isexpressed by the following equation 2.

$\begin{matrix}{V_{n} = \sqrt{\frac{kT}{C_{fd}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

This difference in the reset noise equations 1 and 2 can be understoodby making the analogy with a heat transfer. In the case of the reset tothe voltage source, the charge is supplied from an infinite chargereservoir. The supply of the charge from the infinite charge reservoircan be made analogous to an isothermal process with an infinite heatreservoir. In the case of a reset to a small capacitor, the charge inthe circuit is a quasi constant isolated from the infinite chargereservoir. Thus, the reset to the small capacitor is similar to anadiabatic process. By making a capacitor C_(w) in Eq. 1 much smallerthan the capacitor C_(fd) in Eq. 1, a substantial reduction in the kTCnoise can be expected. Also, when the capacitance C_(w) becomesinfinite, the eq. 1 reverts to the standard case of the reset to thevoltage as can be expected.

This improvement in the kTC noise becomes valuable in the 3T pixelsensors, or in the combination of the shared concept with the 3T and 4Tpixels as mentioned above, which are described in FIGS. 4 and 5.

According to the embodiments of the present invention described in FIGS.4 and 5, the pinned photodiode and the charge transfer transistor arenot included. The signal is directly integrated on the FD node 303.After the integration and the readout, the FD node 303 is reset abovethe Vdd level 329 by the described charge-pumping concept. Since thecharge-pumping reset does not generate the kTC reset noise, it issufficient to read the signal only once and thus, eliminate the CDSprocedure. It may be advantageous to reset the FD node 303 first to aVdd level 329 generating kTC noise and then, pump the charge up to ahigher level to make sure that the process always starts the same waywithout having any memory of the previous pixel signal. This will not bediscussed in any more detail in the detailed description since suchconcepts are well known in the art.

According to the embodiments of the present invention, a novel pixelgenerating a higher dynamic range and a better signal to noise ratio,and having a higher voltage swing and a larger well capacity isdescribed. The novel pixel can be obtained by using the burst reset andincorporating the special potential well into the reset transistor andthe JFET (i.e., the pinned potential barrier) next to the resettransistor. This improvements and innovations are not intended to limitbut to be illustrative. Also, persons skilled in this art can makemodifications and variations in light of the above teaching. It istherefore to be understood that changes may be made in the particularembodiment of the present invention disclosed, which are within thescope and sprit of the invention as defined by appended claims.

According to the embodiment of the present invention, a burst of thereset pulse is used instead of using the single reset pulse added to thegate of the reset transistor. To use the burst of the reset pulse allowsresetting the FD node above the Vdd level. Thus, the photodiode chargecapacity and the FD voltage switching can be simultaneously increased byincorporating the special potential well to the gate of the resettransistor and the special pinned potential barrier between the resettransistor and the N+-type Vdd node. As the result, as compared with theconventional CMOS image sensor, a CMOS image sensor having a largercharge capacity, a higher dynamic range, a larger signal to noise ratio,and a small sized pixel can be fabricated.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaim.

1. A reset transistor, comprising: a floating diffusion region fordetecting a charge; a junction region for draining the charge; a gatefor controlling a transfer of the charge from the floating diffusionregion to the junction region upon receipt of a reset signal; and apotential well incorporated underneath the gate.
 2. The reset transistorof claim 1, further comprising a pinned potential barrier formed betweenthe potential well and the junction region.
 3. The reset transistor ofclaim 1, wherein the potential well is formed on the side of thejunction region inside a substrate underneath the gate.
 4. The resettransistor of claim 2, wherein the pinned potential barrier comprises ajunction field-effect transistor including a gate coupled to asubstrate, one virtual junction region, and another junction regionshared with the junction region.
 5. The reset transistor of claim 1,wherein the reset signal comprises a burst reset signal.
 6. The resettransistor of claim 1, wherein the potential well is an impurity dopedregion disposed inside the substrate underneath the gate.
 7. A pixel ofan image sensor, the pixel comprising: a photodiode; and a resettingdevice comprising a reset transistor, the reset transistor comprising: afloating diffusion region for detecting a charge; a junction region fordraining the charge; a gate for controlling a transfer of the chargefrom the floating diffusion region to the junction region upon receiptof a reset signal; and a potential well incorporated underneath thegate.
 8. The pixel of claim 7, further comprising: a source followertransistor for amplifying a photo-generated charge of the floatingdiffusion region; and an addressing transistor for selecting an outputof the source follower transistor and providing the output of the sourcefollower transistor as a pixel output.
 9. The pixel of claim 7, furthercomprising: a transfer transistor for transferring the photo-generatedcharge of the photodiode to the floating diffusion region; a sourcefollower transistor for amplifying the charge of the floating diffusionregion; and an addressing transistor for selecting an output of thesource follower transistor and providing the output of the sourcefollower transistor as a pixel output.
 10. A shared photo site pixel,comprising: a photodiode coupled to a floating diffusion region inparallel; and a reset transistor for resetting the floating diffusionregion, the reset transistor comprising: a junction region for draininga charge; a gate for controlling a transfer of the charge from thefloating diffusion region to the junction region upon receipt of a resetsignal; and a potential well incorporated underneath the gate.
 11. Theshared photo site pixel of claim 10, further comprising a plurality ofcharge transfer gates for controlling a transfer of the charge from thephotodiode to the floating diffusion region.
 12. The shared photo sitepixel of claim 10, further comprising a pinned potential barrier formedbetween the potential well and the junction region.
 13. The shared photosite pixel of claim 11, further comprising a pinned potential barrierformed between the potential well and the junction region
 14. The sharedphoto site pixel of claim 10, wherein the potential well is formed onthe side of the junction region inside the substrate underneath thegate.
 15. The shared photo site pixel of claim 11, wherein the potentialwell is formed on the side of the junction region inside the substrateunderneath the gate.
 16. The shared photo site pixel of claim 12,wherein the pinned potential well comprises a junction field-effecttransistor including a gate coupled to a substrate, one virtual junctionregion, and another junction region shared with the junction region. 17.The shared photo site pixel of claim 13, wherein the pinned potentialwell comprises a junction field-effect transistor including a gatecoupled to a substrate, one virtual junction region, and anotherjunction region shared with the junction region.
 18. The shared photosite pixel of claim 10, wherein the reset signal comprises a burst restpulse.
 19. The shared photo site pixel of claim 11, wherein the resetsignal comprises a burst rest pulse.
 20. The shared photo site pixel ofclaim 10, wherein the potential well is an impurity doped regiondisposed inside the substrate underneath the gate.
 21. The shared photosite pixel of claim 11, wherein the potential well is an impurity dopedregion disposed inside the substrate underneath the gate.
 22. The sharedphoto site pixel of claim 12, wherein the potential barrier hassubstantially the same pinned potential as the photodiode.
 23. Theshared photo site pixel of claim 13, wherein the potential barrier hassubstantially the same pinned potential as the photodiode.